The A to Z of RCDs14 January 2013

Selecting the right RCD for each situation will help to ensure safety for electrical users within the installation.Residual current devices (RCDs) are essential safety equipment these days. Their application and installation is provided by AS/NZS3000 – Wiring Rules and testing requirements by AS/NZS3017 and AS/NZS3760. Chris Halliday of the Institute of Electrical Inspectors outlines the practical application of RCDs and issues associated testing.

Residual current devices (RCDs) are essential safety equipment these days. Their application and installation is provided by AS/NZS3000 – Wiring Rules and testing requirements by AS/NZS3017 and AS/NZS3760. Chris Halliday of the Institute of Electrical Inspectors outlines thepractical application of RCDs and issues associated testing.

RCDs, or safety switches as they are more commonly known these days, provide essential safety protection against electric shocks. However, they do not negate safe electrical practices or provide guaranteed protection for human life.

A toroidal current transformer is used in the RCD to sense any ‘leakage current’ that might well flow through a human being and a trip mechanism is actuated if the rated residual current rating is exceeded.

Many people think that RCDs can be forgotten once installed but they will only provide ongoing protection if operational. Therefore regular checks/ testing will be required to ensure they are in working order. Mechanical components may ‘stick’ if the RCD is not operated regularly and the electronics used in some versions can be damaged over time by overvoltage, impulsive transients that may occur due to switching or lightning and increased heating due to harmonics or from simply being in a hot metallic switchboard on the western wall of a premise. The typical life of electronic components is approximately 10 years and so RCDs will need replacing over time even if they are operated in a favourable environment.

Type 1, or 10mA, RCDs, seek to protect people in areas of increased risk such as circuits supplying bathrooms and outdoor equipment and for use in kindergartens or medical areas. They should stop a person from ‘locking on’ due to muscular contraction that occurs around the 10mA level of electric shock for adult males (or less for females and children) and should also protect vulnerable people with medical conditions and/or where metallic medical equipment has been introduced into the patient’s body. More circuits will be needed to manage the accumulation of leakage currents when using Type 1 RCDs.

Type 2, or 30mA, RCDs, should operate to protect people by preventing the heart from going into ventricular fibrillation.

We have seen a greater use of RCDs over the last few versions of the Wiring Rules and an increase in requirements can be expected in the future. My motto is: “if in doubt – use a safety switch”.

Wiring Rule requirements

Clause 2.6.3.1 of the Wiring Rules requires the use of RCDs for socket outlets, lighting points and for directly connected hand-held equipment in residential areas. In other installations Clause 2.6.3.2 requires the use of RCDs for socket outlets not exceeding 20A, lighting circuits not exceeding 20A, directly connected hand-held equipment, etc. 10mA RCDs are required for home dialysis in accordance with Clause 2.6.3.3 and AS/NZS3003.

30mA versions are required for fountains and water features in accordance with Clause 6.4.3.2.1, for saunas in accordance with Clause 6.5.3.3 and for refrigeration room heating elements in door seals in accordance with Clause 6.6.4.6.

Socket outlets added to an existing circuit are to be protected by an RCD in accordance with Clause 2.6.3.4 of the Wiring Rules. This does not mean that an RCD has to be fitted at the origin of the circuit but it is much safer to do so as the existing part of the circuit will also be protected.

RCDs provide a means of complying with Clause 1.5.5.3 for the disconnection requirements of not more than 0.4 seconds and for 50mm clearance required by Clause 3.9.4.2 for wires concealed from the surface of a wall, floor, ceiling or roof. The last rule helps ensure that RCDs are fitted to fixed equipment circuits that have never before needed an RCD, e.g. stoves and air conditioners.

Other standards such as AS/NZS3001 for transportable structures and vehicles including their site supplies and AS/ NZS3004 for marinas provide additional requirements for the application of RCDs.

Preventing problems

Nuisance tripping of RCDs is one of the biggest complaints associated with RCDs. Electricians can help to prevent problems by considering the number of socket outlets and the nature of the equipment likely to be connected and thereby ensuring leakage currents are typically less that 1/3 of the residual current rating of the RCD.

Arranging circuits in accordance with Clause 2.6.2.4 of the Wiring Rules will also help minimise nuisance trips and effects. No more than 3 final sub-circuits on an RCD are permitted but it is much better to have an RCD per circuit. Splitting lighting circuits across RCDs will help minimise risks due to the lack of light if one circuit trips.

Impulsive transients, such as caused by lightning or switching, can cause nuisance tripping of RCDs. Surge Protective Devices (SPDs) are recommended to be installed to prevent nuisance trips and to protect electronic type RCDs as well as the other equipment within the installation. Clause F1.2.2, from Appendix F of the Wiring Rules, suggests that SPDs should be installed after the main switch but before RCDs. Clause F1.2.4 provides additional requirements should the SPDs be connected after RCDs.

Minimising the length of circuits helps in reducing high frequency voltages being coupled into the circuit and causing nuisance tripping.

Selecting the right RCD

The following things need to be considered when selecting the right RCD for any situation:

• Short circuit rating, e.g. 10kA;• Rated load for residual current operated circuit-breaker without integral overcurrent protection (RCCB) or overcurrent rating if using a residual current operated circuit-breaker with integral overcurrent protection (RCBO), e.g. 20A;• Residual rating, e.g. 10mA;• Coordination with downstream RCDs - consideration of the residual rating and use of a ‘selective delay’ will help to ensure coordination;• D.C. sensitive RCDs – Type A or Type B. This type of RCD is used where electrical switchgear and control gear connected on the circuit is comprised of rectifier devices such as diodes, triacs, etc;• The action of the RCD in the event of loss of supply – Type FS. These RCDs automatically open on failure of the supply voltage or continue to provide protection;• Applied voltage e.g. 230V;• Frequency rating e.g. 50Hz;• Available space – are single pole units required because of limited room on the switchboard;• Replacement RCDs may need to be the same make/brand to fit in existing switchboards.

Additional protection

The use of an RCD as a main switch (non-mandatory) has been recommended in the 2007 version of the Wiring Rules for domestic installations at Clause 2.6.2.3 to prevent the initiation of fires. A ‘Type S’ RCD is recommended with a rating of between 100mA and 300mA. The Type S provides a ‘selective delay’ to help ensure coordination with downstream RCDs. It also provides some backup should a downstream RCD fail. Whilst this rule has been in place for about five years, it appears that few electricians have recommended them to their clients.

Testing

Tests shall be performed on each final sub-circuit protected by an RCD to verify that the RCD operates to disconnect the designated circuit as detailed at Clause 8.3.10 of the Wiring Rules. This clause provides for testing by the integral push button or by use of special test equipment. Test equipment used for testing RCDs is now quite sophisticated and will test an RCD over a wide range of tests.

Many electricians are now aware that RCDs not only protect for active to earth faults but also for neutral to earth faults or swapped neutrals. Therefore insulation resistance testing from neutral to earth is now additionally important.

The test and tag standard, AS/ NZS3760, and AS/NZS3017 provide maximum trip times of 40ms for Type 1 RCDs and 300ms for Type 2 RCDs. Push button tests by the user should be carried out typically every three or six months and by operating time and push button every 12 to 24 months. Check the relevant standard for requirements that apply for any particular situation or

location. Don’t forget to apply a test tag to the RCD or adjacent to it once it passes the appropriate testing.

Codes of Practice or state based requirements may require more stringent requirements and frequent testing than those provided by the test and tag standard or AS/NZS3017. It will pay to check requirements in the state you’re operating within.

Arc flash is an issue that can’t be ignored. Switching and testing become the real risks if you have a policy of not working live. Testing of RCDs can be an arc flash issue unless much of the risk is designed out. An outlet on switchboards for each circuit or a single outlet switchable from all RCD protected circuits will minimise the risks when testing RCDs.

Staying out of court

RCDs are now essential safety equipment. Once identified as faulty, an RCD will need to be isolated, locked and tagged out if it cannot be replaced immediately. There have been numerous incidents over the last few years where this hasn’t occurred. This places lives and electrical contractor insurance policies and license at risk, as well as leading to prosecutions and fines from state based electrical safety regulators.

Conclusions and summary

RCDs are essential safety equipment. If faulty, the RCD must be isolated, locked and tagged out if not replaced immediately.

RCDs will need ongoing care including push button testing and trip timing tests for businesses. Replacement of RCDs will be required in time due to electrical and environmental conditions and the aging of electronic components.

The Wiring Rules, other relevant standards and state based legislation, codes and guides have extensive requirements in regards to the installation and testing of RCDs. Electricians need to be fully aware of these requirements.

Nuisance tripping of RCDs can be minimised by the wiring practices of the electrician.

The risk of arc flash associated with testing RCDs is best designed out by providing a socket outlet(s) for testing purposes on the switchboard.

Selecting the right RCD for each situation will help to ensure safety for electrical users within the installation.

All in all, the issues associated with RCDs are somewhat detailed but electricians must be fully aware of all requirements to ensure safety and minimise risks.